6GK6 has the G3 run out to pins 1 and 9 whereas the 6BQ5 has it internally connected to the cathode (pin 3).

The anodes, Grid 1 and heaters are all the same (pins 2,4,5,7)

The screen grid of 6BQ5 is pin 9, change this to pin 8 for the 6GK6

The cathode of 6GK6 is pin 1 whereas it is pin 3 for 6BQ5
BUT pins 3 and 9 of the 6GK6 are Grid 3 which in the 6BQ5 is an internal connection back to pin 3. Just link pins 1 and 9 to pin 3 for the 6GK6.